1. Field of the Invention
The present invention is directed to a dry friction damper for damping vibrations in a body subject to cyclic energy.
2. Discussion of the Related Art
Numerous techniques have been proposed for the damping of vibrations in a body. One known technique involves constraining an elastic damping layer between two rigid layers to form a laminate. Upon the laminate being subject to vibrations, the resulting deformation of the laminate gives rise to damping as energy is absorbed in the soft intermediate layer. An example of such constrained layer damping may be found in U.S. Pat. No. 3,071,217 to Gould.
Related to the above damping technique is the damping or vibration isolation technique of mounting a vibrating member on a vibration isolator, such as a rubber mount. This is often used in automobile suspensions and is illustrated in U.S. Pat. No. 2,717,747 to Rosenzweig.
Also known is hydraulic damping in which energy is absorbed by forcing an incompressible fluid through an orifice. This is illustrated in U.S. Pat. No. 4,288,063 to Brenner et al.
Several dry friction damping techniques are also known. One type involves the relative movement of an elastic element relative to a rigid element, with damping resulting from compression and shear of the elastic element, as well as by slipping friction of the elastic element relative to the rigid element. This is shown in U.S. Pat. No. 2,925,973 to Aebersold.
Yet another form of dry friction damping, known as Coulomb friction damping, arises due to the relative sliding of two smooth, flat rigid surfaces, rather than as a result of the distortion of an elastic element. It has been known, for example, to use Coulomb friction in the damping of cantilever beams, such as turbine blades. This is described in "Friction Damping of Resonant Stresses in Gas Turbine Engine Airfoils, J. H. Griffin, ASME Paper No. 79/GT-109, and is generally illustrated in FIG. 1. There, dry friction damping is achieved by the provision of a link 6 fixed to a vibrating point 8 on a gas turbine blade 4. The distal end 10 of the link 6 is held pressed against a surface 12 of a relatively rigid structure such as a cover plate of the engine. The normal pressing force of the distal end 10 of the link 6 against the surface 12 is provided by the inherent resilience or springiness of the link itself. Upon the blade 4 being subject to vibrations, relative slip occurs between the surface 12 and the distal end 10, whereby energy dissipation results. Such slippage occurs when the load on the link due to the vibration exceeds the friction force .mu.N, wherein .mu. is the coefficient of friction between the link and the surface 12, and N is the normal force. It may be appreciated that the slip occurs in a direction 14 which is parallel to the principal vibrational direction of the blade.
However, such dry friction dampers which rely on Coulomb friction damping by point or small area contact between a vibrating member and a relatively stationary surface have been found to be ineffective at high vibration levels when slip occurs in a direction which is parallel to the principal vibrational direction.
U.S. Pat. No. 4,516,658 to Scarton et al discloses the uses of Coulomb damping by laying a flat plate of extensive area on a plate to be damped. Such a technique, however, is not effective for many damping applications, such as, for example, the damping of a vibrating beam such as a turbine blade.
The ASME Paper 81-DET-139, "Structural Damping by Slip Joints", L. Jezequel, concludes from theoretical analysis that a Coulomb friction mechanism leads to an equivalent linear viscous damping for large motions of a circular plate when slip occurs in a direction which is perpendicular to the principal vibrational direction.